1. Field of the Invention
The present invention relates generally to hip prostheses and more specifically to an interlocking reverse hip prosthesis allowing increased range of motion and stability during excessive range of motion.
2. Description of the Prior Art
It can be appreciated that several hip implants have been in use for years. Typically, conventional hip implants comprise a femoral component having an articular femoral ball attached to a stem. A femoral stem is inserted into the medullary canal of the femur after preparation and reaming using appropriate reamers by the operating surgeon. The stem can be secured with bone cement or press fit. An acetabular component having the shape of a cup is inserted into an acetabular socket after preparation and appropriate reaming and secured with cancellous screws through holes in the cup. It can also be secured with bone cement or press fit or a combination thereof.
The acetabular cup is metallic and it is internally lined with high-density polyethylene or ceramic. Said lining is secured into the acetabular cup by a press-fit mechanism.
The main problem with conventional hip implants is the instability of the prosthesis at the extreme range of motion, thereby allowing the articular femoral ball to dislodge and dislocate. Prior art teaches constrained and preassembled ball and cup devices or devices wherein the ball and cup members are implanted separately whereupon the ball element is forced into a resilient opening in the cup and thereafter held in place by the resilient material. Other constrained acetabular cups may include a locking ring such as the one described by Albertorio et al. U.S. Pat. No. 6,527,808. In the case of cup elements having retaining rings, the ball member is forcefully inserted into the cup after the two elements are implanted. This constitutes a weak link where forces exerted on the prosthesis by ambulatory motion may exceed the forces used to assemble the implant thereby causing the ball to be separated from the cup.
While these devices may be suitable for the particular purpose which they address, they do not provide an interlocking mechanism as in the reverse hip implant design of the present invention. The very nature of applicant's design allows increased range of motion and increased stability at extreme ranges of motion thereby reducing the risk of dislocation.
In these respects, the interlocking reverse hip prosthesis according to the present invention substantially departs from the conventional concepts and designs of the prior art because the articular femoral ball of the prior art is replaced with an articular femoral cup and the acetabular cup is provided with an acetabular ball. Thus an apparatus is provided which is primarily developed for the purpose of reducing the risk of dislocation of hip implants at extreme ranges of motion.
Furthermore, since the articular surface of the femoral cup of the invention is fully in contact 100% of the time with the surface of the acetabular ball, it is clear that this will improve tribology because the weight bearing distribution is improved on the articular surfaces, thus decreasing the wear of the surfaces in contact and reducing the risk of wear particles being released in the joint. The later, being very detrimental to the proper function of the joint.